The invention relates to a molybdenum-based nanocomposite. More particularly, the invention relates to an x-ray tube having an x-ray target comprising a molybdenum-based nanocomposite. Even more particularly, the invention provides a method for making molybdenum-based nanocomposites for x-ray applications.
X-ray tubes generate x-rays by bombarding a layer of an x-ray target material with high energy electrons. The target comprises elements with high atomic number (such as tungsten and rhenium) and is attached to a substrate disk comprising a refractory metallic material having a high thermal conductivity. To dissipate the intense heat generated by the electron bombardment, the target disk is rotated at speeds in excess of 8400 rpm. Additionally, the high-conductivity target disk conducts the heat to a graphite block, which acts as thermal storage material. In medical diagnostics, the demand for ever-improving x-ray image quality in conjunction with the need for computer tomography (CT) systems to perform high-speed cardiac imaging necessitates the use of high peak power (in excess of 70 kW), and high target rotation speeds, which increase the thermal and structural loading requirements of the target material well beyond current capabilities. Thus, there is a need for target materials with high strength and creep resistance to meet the thermal and structural demands generated by the use of high peak power and high rotation speeds.
The continuing effort to design and build more powerful and more efficient x-ray tube components requires the use of materials having enhanced high temperature performance capabilities. Such performance enhancements require state-of-the-art materials with vastly improved mechanical properties such as strength, creep resistance, and thermal stability.
For x-ray tube and other applications, high temperature structural materials can be strengthened in a number of ways such as, for example, grain refinement, solid solution strengthening, precipitate strengthening, composite strengthening, and dispersoid strengthening. One method of strengthening alloys known as Orowan strengthening incorporates a fine distribution of hard particles into a metallic alloy matrix. Orowan strengthening depends upon the formation of an array of dispersoid particles that serve as obstacles for impeding dislocation motion within the alloy matrix. The strength of these particle-reinforced alloys is inversely proportional to the spacing between these particles, which can be controlled in turn by controlling the size of the dispersoid particles. Thus, the use of nanoparticles as dispersoids offers the potential of substantially enhancing alloy strength.
The introduction of hard dispersoid nanoparticles during the processing of the nanodispersoid-reinforced alloys presents a technical challenge. Current processes to disperse particles include powder metallurgy routes, such as mechanical alloying of micron-sized particles, in combination with secondary processes, which include hot-isostatic pressing and thermomechanical processing by hot-forging or extrusion. In the mechanical alloying process, nanoparticles are created by repeated fracture of the micron-size dispersoid particles during milling. While this is a well-established process for oxide-dispersion strengthened (ODS) alloys in iron- and nickel-based alloys (such as, for example, Inconel MA alloys), the process fails to produce a homogeneous distribution of the particles in the molybdenum-based matrix, especially for large components. In addition, the loading of the particles in the alloy composites produced by this process is typically limited to less than 2 percent by volume.
Current processes are unable to produce alloy nanocomposites having sufficiently high loadings of nanoparticles. Therefore, what is needed is a molybdenum-based nanocomposite in which dispersoid nanoparticles are homogeneously distributed throughout the molybdenum-based matrix. What is also needed is a molybdenum-based nanocomposite having a sufficiently high loading of dispersoid nanoparticles having high temperature performance capabilities that are adequate for use in x-ray target assemblies. What is further needed is a method of making molybdenum-based nanocomposites having high loadings of dispersoid nanoparticles, wherein the dipersoid nanoparticles are homogeneously distributed throughout the alloy nanocomposite.